Mast-mounted vibration isolators are well-known in the art for canceling or substantially reducing vibratory forces active on a helicopter rotor. While most such devices are referred to as "vibration absorbers", this may be viewed as a misnomer inasmuch as these devices typically isolate the energy produced by cyclic in-plane and out-of-plane loads rather than absorb the energy as the name implies. Such devices typically comprise: a hub attachment fitting for mounting to the main rotor hub such that the isolator is rotated in a plane parallel to the main rotor disc, and a spring-mass system mounted to and rotating with the hub member. The troublesome in-plane forces comprise (n-1) frequency vibrations and (n+1) frequency vibrations. By (n-1) vibrations we mean the vibrations which oscillate at a frequency equal to the number of blades (n) minus 1 times rotor rpm, i.e. (n-1) * rotor rpm, and by (n+1) vibrations we mean the vibrations which oscillate at a frequency equal to the number of blades (n) plus 1 times rotor rpm, i.e. (n+1) * rotor rpm. Taking a four-bladed rotor as an example, these vibrations are also sometimes referred to as 3P and 5P vibrations. The spring arm-mass system is tuned in the non-rotating condition to a frequency equal to n * rotor rpm (e.g., 4P for a four-bladed rotor) at normal operating speed, so that in the rotating condition it will respond to both N+1 and N-1 frequency vibrations (i.e., 3P and 5P).
FIGS. 1a, 1b and 1c, depict a prior art vibration isolator similar to that described and illustrated in U.S. Pat. No. 5,901,616 to Miner et al. (hereafter `Miner` and assigned to the assignee of the present invention) which is hereby incorporated by reference. As shown a vibration isolator 100 comprising a circular inertial mass 102 is supported from an inner hub 104 by resilient spring arms 106 so as to be capable of oscillation in any direction within its plane of rotation. The isolator 100 of FIG. 1a is mounted to the rotor mast 108 of FIG. 1b to render the configuration of isolator/mast 110 of FIG. 1c. This prior art isolator 100 is capable of canceling both (n-1) and (n+1) frequency vibrations of a helicopter rotor in a single installation. As shown in FIG. 1a the spiral shaped spring arms 106 are fastened to both the inner hub 104 and the circular inertial mass 102 with bolts at each of the spring arm ends 112 and 114. These spring arms 106 are typically made from graphite composites consisting of multiple parallel layers of graphite held together by epoxy as is well known in the art.
The root end retention of these composite spring arms 106 which are incorporated in main rotor helicopter vibration isolators are most highly loaded and structurally the most critical in the isolator design. The operation of the isolator creates a high concentrated bending moment that must be reacted at the retention areas 116 and 118 by a bolted connection consisting of a metal hub (inner metal hub 120 and outer metal hub 122) and respective metal retention plates 124, 126 (FIG. 1a). This retention configuration creates a very abrupt load transfer from the composite spring arm 106 to the bolted metal retention at both inner and outer retention areas 116, 118. A severe prying of the spring arm under the bending load generates very high transverse shears and corresponding interlaminar shear stresses which are the most critical loading for a composite isolator spring arm.
Previous main rotor isolator designs, such as Miner and Vincent et al. (U.S. Pat. Nos. 4,145,936 and 4,225,287 ) relied upon a bolted rigid spring-to-hub connection. This type of design configuration generates a very sudden load transfer of the isolator spring arm bending moment into the rigid metal retention, creating very high interlaminar shear stresses in the composite spring arm. Since a given composite material selected for the spring arm design has a specific interlaminar shear stress strength, a higher spring arm stress must be reduced by either thickening or widening the spring arm geometry which compromises the design to other than optimum.
While the teachings disclosed in the Miner and the Vincent patents provide a baseline for design and development of the vibration isolator described therein, they do not address the issue of reducing interlaminar shear stress at the root retention area of the spiral shaped springs.
Without reduction of this shear load, the springs must be over built throughout to compensate for this factor, increasing cost and weight; both important factors in helicopter design.
A need, therefore, exists for an apparatus and method of attaching spring arm ends to the inner and outer hubs of a vibration isolator which reduces interlaminar shear loads, and, inter alia, facilitates optimum design requirements for a composite spring arm, provides improved structural efficiency, reduces fabrication costs, and reduces the weight of a helicopter.